1. Field of the Invention
The present invention relates to a high-speed wireless access device, in particular, to a high-speed wireless access device, which reduces power consumption during the stand-by mode of wireless modems in a terminal in a wireless asynchronous transfer mode (ATM).
2. Description of the Related Art
There are several access systems like that illustrated in FIG. 9 in which wireless terminal access to a hard-wired LAN or public phone network via base stations(BS). Typical TDMA(Time Division Multiple Access)/TDD(Time Division Duplex) systems thereof are exemplified by the configuration illustrated in FIG. 10 and a corresponding distributed system in FIG. 11. In a TDMA/TDD system as exemplified by FIG. 10, every mobile terminal (MT) performs timing synchronization by using a preamble signal at the beginning of a frame added by a base station. Then a mobile terminal determines whether or not a packet addressed to that particular mobile terminal exists by checking a control packet following the preamble signal. If the packet signal to such a mobile terminal exists, this packet signal is loaded from any one of the packets (P1 . . . Pk) following a control packet. In corresponding distributed systems exemplified by FIG. 12 also signal data is loaded after determining that a received packet is addressed to a mobile terminal. However, there is a difference between the two systems, namely, a TDMA/TDD system requires for only one preamble signal to be added to the beginning of a downlink frame from a base station, while corresponding distributed systems should add respective preamble signals to the beginning of each of packets from and to a mobile terminal. Accordingly, although corresponding distributed systems allow an "ad hoc" communication between mobile terminals, effective data transmission throughput may be decreased depending on data length.
When multimedia services including such image signals as MPEG2 images are provided by access systems such as those as mentioned above, the data transmission rate in a wireless section should be not less than 20 M bps. This is due to existing overheads in regard to preamble signals and the transmission rights control. In addition, if mobile operations are to be considered, wireless transmission systems should allow high-speed multimedia transmission under a multipath fading condition. Accordingly, there are devised systems which prohibit receiving of long-delayed multipath signals by using sharp directional antennas, or equalizing systems which remove an intercede interference by multipath. Sharp directional antennas have an overwhelming advantage for communication between fixed stations, but if any one of the stations is a mobile terminal, complicated and troublesome antenna control is required, hence equalizing systems are currently considered to be advantageous. An appropriate equalizing system against multipath fading may be a system which uses a decision feedback equalizer (DFE) as a on-linear equalizer or a maximum likelihood system estimation type of equalizer, or their combination. However, each system has disadvantages of requiring large-scaled circuits and much power consumption. For example, under some conditions with a delay spread indicating a spreading state of multipath about 20 ns maximum, if high-speed transmission with a symbol rate of 25M symbols/sec is performed, an equalizer should have not less than 10 taps, which results in a large-scaled circuit.
FIG. 8 illustrates a block diagram of a DFE as an example equalizer. Further, FIG. 13 illustrates an example of the structure of a preamble when using an equalizer. In FIG. 8, reference numeral 301 denotes a feed-forward type filter. Similarly, 302 denoting a feedback filter, 303 denoting a discriminator, 304 denoting a parameter estimator, and 305 and 306 denoting adders. Further in FIG. 13, there are four preamble signals, namely preamble signal 211 for automatic gain controller (AGC), preamble signal 212 for phase synchronization, preamble signal 213 for timing extraction, and preamble signal 214 for tap coefficient setting. Reference numeral 215 denotes a unique word (UW) in order to identify the beginning of data, and reference numeral 220 denotes data item.
In FIG. 8, feedback filter 302 is operated according to a symbol rate. If, however, feed-forward filter 302 is also operated at such symbol rate, there may be occurred aliases, which cause intercode interference against adjacent symbols of both sides. In this case, intercede interference to such adjacent symbols should be excluded through precise timing extraction by using preamble signal 213 for timing extraction as shown in FIG. 13. However, since receiving signals are significantly distorted under multipath fading conditions, it is difficult to carry out precise timing extraction. In this connection, modems for hard-wired low-speed data communication commonly use a method for avoiding aliases in which a sampling is performed at a frequency double of a symbol rate ("double frequency sampling"). Samplings may be carried out at the same frequency as that of a symbol rate. In such a case it is sufficient to use half the number of taps of feed-forward filter 301 as compared with the double frequency sampling, thereby allowing circuit size to be minimized. However, sampling at the same frequency as the symbol rate as described above requires preamble signal 213 for timing extraction, and time of existence of such a preamble signal 213 correspondingly shortens data transmission rates, which results in less-effective data transmission throughput.
On the other hand, when sampling at a frequency double of a symbol rate is carried out, advantageously effective data transmission may be obtained, since preamble 213, for timing extraction, is no longer required. However, large-scaled circuits should be prepared for feed-forward filter 301, and much power consumption is still required in order to activate feed-forward filter 301 at a double clock frequency (for present example, for instance, at 50 MHz). As discussed above, an equalizer for high-speed transmission has disadvantages of large-scaled circuits and much power consumption. Accordingly, either the TDMA/TDD system or corresponding distributed system consumes much electric power it the equalizer is operated throughout the stand-by mode, and such a system is not suitable for wireless multimedia access services on the assumption that terminals are powered by battery. In addition, when an equalizer is used, preamble 214, for tap coefficient setting, should be included in addition to preamble 211 for AGC, 212 for phase synchronization and 213 for timing extraction (as described above), thereby preamble length may become longer than that of a low-speed wireless modem. Accordingly, depending on the value of data length 220, there may be a case that the data transmission becomes significantly less effective and only lower speed data service can be provided.
Wireless modems used for high-speed wireless access systems should satisfy two requirements, namely, providing lower power consumption and securing effective data transmission throughput. Therefore, in regard to wireless modems suitable for high-speed wireless transmission and consideration of mobile operation, there should be provided a means of low-speed demodulation to reduce power consumption during the stand-by mode, as well as high-speed modulation/demodulation including an equalizer to assist operability against multipath fading conditions.
The Japanese Patent Laid-Open Publication No. Hei 2-5633 discloses a means for reducing power consumption as illustrated herein in FIGS. 14, 15 and 16. Shown in FIG. 14 is a block diagram of a receiver, and in FIGS. 15 and 16 structures of frames. The receiver has a low-speed clock generator 91 and a high-speed clock generator 92, by which calling words are received at a low-speed clock during the stand-by mode. FIG. 15 shows a state, in which a call group that such station is belonging to is received; a message is received by changing clock rate from the lower one to the higher one. FIG. 16 shows a state that, in addition to a call group that such station is belonging to, the clock is switched to high-speed to receive the message if the call word includes a call sign addressed to such station. In such described arrangement, power consumption during the stand-by mode may be decreased.
However, in regard to the means disclosed in Japanese Patent Laid-Open Publication No. Hei 2-5633, when a packet addressed to that station is received, there is provided only a means of switching to a high-speed clock in order to decode the error correction encoded high-speed data. Intercode interference due to multipath fading cannot be removed only by an error correction function, thus high-speed wireless access is hard to accomplish. Thus, it is necessary, as described above, to reinforce operability against multipath fading conditions by using for example an equalizing system and so forth, under such multipath fading conditions.
Therefore, a wireless modem should be provided with a means for reducing power consumption during stand-by mode by using low-speed demodulation as well as a built-in equalizer for high-speed modulation/demodulation. A corresponding frame format is that as illustrated in FIG. 17, in which a low-speed packet 100 is added to the beginning of high-speed packet 200. FIG. 13 illustrates the structure of the high-speed packet 200, comprising high-speed preamble signal 210 and data section 220, and FIG. 7 illustrates the structure of a low-speed packet 100. When comparing a frame format such as that illustrated in FIG. 17 with that of FIG. 16, it is clear that the frame format illustrated in FIG. 17 should include a high-speed preamble signal 210 as well as a low-speed mode preamble signal 110, thus effective throughput becomes decreased. In this connection, a TDMA/TDD method such as that shown in FIG. 1 may offer better throughput since it is sufficient to respectively add a low-speed control packet and a low-speed-mode preamble to the beginning of a downlink frame. However, access by a corresponding distribution system, such as that illustrated in FIG. 12, used for example in a wireless LAN operating in the 2.4 GHz frequency band, may cause effective throughput to become significantly decreased depending on the length of high-speed data.